In view of the recent instability of the price of crude oil and natural gas, there has been renewed interest in alternate sources of energy and hydrocarbons. Much of this interest has been centered on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal, and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast reserves of hydrocarbons in the form of oil shales exist throughout the United States. The Green River formation of Colorado, Utah, and Wyoming is a particularly rich deposit and includes an area in excess of 16,000 square miles. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States, almost sixty percent located in the Green River oil shale deposits. The remainder is largely contained in the leaner Devonian-Mississippi black shale deposits which underlie most of the eastern part of the United States.
Oil shales are sedimentary inorganic materials that contain appreciable organic material in the form of high molecular weight polymers. The inorganic part of the oil shale is marlstone-type sedimentary rock. Most of the organic material is present as kerogen, a solid, high molecular weight, three-dimensional polymer which has limited solubility in ordinary solvents and therefore cannot be readily recovered by simple extraction.
A typical Green River oil shale is comprised of approximately 85 weight percent mineral components, of which carbonates are the predominate species. Lesser amounts of feldspars, quartz, and clays are also present. The kerogen component represents essentially all of the organic material. A typical elemental analysis for Green River oil shale kerogen is approximately 78 weight percent carbon, 10 weight percent hydrogen, 2 weight percent nitrogen, 1 weight percent sulfur, and 9 weight percent oxygen.
Most of the methods for recovering kerogen from oil shale involve mining the oil shale, grinding it, and thermally decomposing (retorting) the ground oil shale. In view of the fact that approximately 85 weight percent of the oil shale is mineral components, unless something is done to remove these minerals, most of the oil shale which is fed, heated up, and circulated in a retort is composed of material that cannot produce oil. This high percentage of inorganic material significantly interferes with subsequent shale processing to recover the kerogen. For example, in retorting the shale, either large or numerous retorts are needed to process the commercial quantities involved. Moreover, a substantial amount of heat is expended and lost in heating up the inorganic minerals to retorting temperatures and cooling them back down again.
Another problem associated with the presence of a large amount of mineral matter in the oil shale is pollution. In the retorting process, contaminating fines are produced and must be disposed of. The greater the quantity of minerals, the greater the quantity of polluting fines. Another source of pollution is the spent shale recovered from the retort. During retorting, chemical reactions occur in the shale as the kerogen is volatized. This results in a residue of chemical compounds. Such compounds can present a hazard in surface water pollution after they have been discarded.
As a result of the problems associated with the high percentage of minerals in oil shale, it can be economically beneficial to reject these minerals prior to retorting. This is called "shale beneficiation." This beneficiation is basically divided into two steps: (1) liberating the kerogen from the mineral matter, and (2) separating the kerogen from the mineral matter.
An essential part of liberating the kerogen from the mineral matter is comminuting the oil shale. There are several options for comminuting the oil shale. Hazemag mills, semi-autogenous (SAG) mills, balls mills, and tower mills can be effective equipment for comminution. The number of comminuting stages and the selection of the most efficient mill depends upon the intrinsic grain size of the kerogen and the extent of kerogen liberation required.
In a SAG mill, which is a cascade mill in which about 10 volume percent steel balls supplement the oil shale solid feed as comminution media, the shale can be comminuted down to about 1/2 in. top size. A ball mill, which is a tumbling mill using about 50 volume percent steel balls as comminuting media, can comminute the shale down to about 0.003 in. top size. To obtain a top size of less than 0.003 in., a tower mill can be used. A tower mill is a stirred ball mill that uses attrition as the mechanism for size reduction.
After comminuting the oil shale to produce kerogenrich particles and mineral-rich particles, the second step of beneficiation is separating these particles from each other. The two basic methods of making the kerogen-rich/mineral-rich particle separation are chemical and physical separation.
Chemical separation includes leaching of minerals, such as acid leaching of carbonates, or extraction of kerogen by chemically breaking the kerogen bonds. U.S. Pat. Nos. 4,176,042 and 4,668,380 are examples of chemical beneficiation.
One type of physical separation is density separation. This type of physical separation is possible because kerogen has a specific gravity of about 1 gm/cm.sup.3 and because mineral components in oil shale have a density of about 2.8 gm/cm.sup.3. One type of density separation is heavy media cyclone separation. Heavy media cyclone is a process for separating, by density, relatively coarse shale particles. An example of a heavy media separation process can be found in U.S. Pat. No. 4,528,090. In general, the aim of heavy media separation is to separate shale into a kerogen-rich fraction having low density and a kerogen-lean fraction having high density. The liquid medium used is a mixture of water and finely ground magnetite and ferrosilicon. By varying the concentration of the magnetite and ferrosilicon, the medium can be made to have a density from 1.8-2.4 gm/cm.sup.2 so that the shale can be split at the density required. The kerogen-rich material floats to the top and is taken overhead, and the kerogen-lean material goes into the underflow from the cyclone. The disadvantages of this process are that it relies upon an inherent natural heterogeneity among oil shale particles and that it has not been successful in separating small oil shale particles.
Another type of physical separation is surface property separation. An example of surface property separation is froth flotation. In the froth flotation process, oil shale particles are mixed with an aerated aqueous solution. Since the kerogen-rich particles have greater hydrophobic character than mineral-rich particles, the kerogen-rich particles preferably attach onto air bubbles, thereby causing the kerogen-rich particles to float. Subsequently, the froth containing these kerogen-rich particles is removed. Additives can be used to improve kerogen grade and recovery. One disadvantage of the froth flotation process is the oil shale is required to be comminuted to a fine particle size prior to froth flotation. Another disadvantage of this process is that the effects of different types of collectors, frothers, and dispersants are difficult to predict. In addition, floated, kerogen-enriched shale has a tendency to have a higher concentration of carbonates than starting shale. An example of a froth floation process is disclosed in U.S. Pat. No. 4,673,133.
Another example of surface property separation is selective agglomeration. Selective agglomeration is the combination or aggregation of specific particles into clusters of approximately spherical shape. Selective agglomeration of coal fines is known in the art. Selective agglomeration of high-rank coals using high-quality oils is disclosed in U.S. Pat. Nos. 4,209,301 and 4,153,419. U.S. Pat. No. 4,726,810 discloses a process for selectively agglomerating low-rank sub-bituminous coals using low-quality oil. The difference between the methods disclosed in these patents and the instant invention is that the instant invention selectively agglomerates oil shale rather than coal. Because of the difference in chemistry of oil shale and coal, the methods of selective agglomeration must be different. Coal is typically precomminuted in water; however, precomminuting oil shale in water will interfere with the selective agglomeration of the kerogen.
A form of selective agglomeration used for beneficiating oil shale is kerogen agglomeration. In kerogen agglomeration, shale particles are mixed with an organic liquid and water to form agglomerates of the kerogen-rich particles while the mineral-rich particles disperse into a water phase.
In Reisberg, J., "Beneficiation of Green River Shale by Pelletization," American Chemical Society (ASCMC8), V. 163 (Oil Shale, Tar Sands, and Related Materials), pp. 165-166, 1981, ISSN 00976156, a form of kerogen agglomeration of oil shale is disclosed. This reference describes precomminuting the shale to a size small enough to pass through a screen size of 0.0059 in. (100 mesh). This shale is subsequently comminuted in the presence of heptane and water to form a kerogen-enriched fraction in the form of discrete pellets and a mineral-rich fraction dispersed in an aqueous phase. These pellets are subsequently separated from the aqueous phase using sieves. This process was found to be uneconomical due to the major cost of the power used to pregrind the oil shale prior to kerogen agglomeration. An estimated total comminution power input for this process is 130 Kw-hr/ton of shale.
The Reisberg reference's requirement that the shale be precomminuted to less than 0.0059 in. (100 mesh) prior to kerogen agglomeration is illustrative of the commonly held belief that in order to form agglomerates the shale must be finely precomminuted or prepulverized prior to kerogen agglomeration. Another example of this requirement can be found in U.S. Pat. No. 4,506,835.
The cost of comminuting the oil shale to a fine size prior to kerogen agglomeration has been a major impediment to the development of a commercial kerogen agglomeration process. There is a need for a commercially viable kerogen agglomeration process that separates kerogen from minerals without comminuting the oil shale to a fine size prior to kerogen agglomeration.